Fix pairwise inference task index selection

[SergeiNikolenko]

Description

Fix pairwise inference dataframe generation to use explicit task index selection (task_idx) and add regression tests for single-task and multitask pairwise paths.

Status

• Ready to go

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[SergeiNikolenko]

Closing per author request during fork cleanup/reset.

[hmacdope]

@SergeiNikolenko does this have a specific task that is not working or issue that this is associated with? I wasn't aware that anything was wrong.

[SergeiNikolenko]

Hi, yes, this is fixing a real bug in the pairwise inference path.

Before this change, _generate_pairwise_df was using predictions[:, j], where j came from the generated pair (i, j) rather than from the task index. That means the selected prediction column depended on the pair indices instead of the output task.

In practice, that could lead to two problems:

1. for multitask pairwise inference, predictions could be written from the wrong output column;
2. for single-task pairwise inference, this could hit an out-of-bounds column index depending on the generated pairs.

I added tests covering both the task index selection and the pairwise inference path.